The citrus fruits tend to neutralize too high acidity of the blood, increasing its alkalinity. For this reason, also, they are best before a meal, particularly before breakfast; they have a more laxative and cleansing effect if eaten before the other food. The custom has been, however, to eat fruits after dinner for dessert and they are so given in the following menus. Table XI (page 207) gives the total amount of protein, carbohydrate, and fat needed daily for the work of the body. The method of determining the number of calories produced by each variety of food is also given on page 208. By a little study of the food one ordinarily eats in connection with this tabulation and the tables given on pages 233 to 241, it can be determined whether the food taken each day is well or illy balanced and whether one is eating too much or not enough. Table XIII (page 209) gives the balanced supply for a day of the most commonly used foods and may be consulted as a basis from which to work in constructing balanced meals. Because of the wide variation in methods of preparing food in the home, an exact and absolute standard cannot be fixed. All foods contain combinations of mineral salts, particularly calcium (lime), sodium, magnesium, and potassium. In each food, however, some mineral predominates. For instance, potatoes contain both calcium and potassium but the potassium content is larger than the calcium. For this reason when potassium salts are needed in a diet, potatoes and other potassium-containing foods make a valuable contribution. When potassium needs to be limited these foods should be omitted from the diet. When calcium is needed, as in growing children, calcium-containing foods should be made a large part of the diet. In conditions of health the construction of a balanced diet is a comparatively simple matter. In conditions of disease, however, the question of diet is often one that can only be solved by a skilled dietitian, after a chemical analysis. Unfortunately, the number of these in the United States is not large and their services are not available in many cases in which they are needed. A diet in which the acid-forming elements are in excess will ultimately result in a lessening of the alkalinity of the blood. The blood then, to maintain its balance, withdraws alkaline substances from the tissues. A balance must, therefore, be maintained between the acid and alkaline foods. This has a bearing on scurvy and also in gout. Foods which are called acid, that is, they tend to lessen the normal alkalinity of the blood, are, oats, barley, beef, wheat, eggs, rice, and maize. When the proportion of acid in the blood is too great the supply of these foods should be lessened. Alkaline foods, or those which leave no acid residue, are carrots, turnips, potatoes, onions, milk, blood, peas, lemon and orange juice, and beans. These may be used when there is too much acid in the system.